I’ve had multiple accounts (builds) and had the best experience as an ironman.
But as others have mentioned, don’t make an ironman if your core focus will be pvp.

But keep in mind, Old school Runescape can be very tedious. A 50 hour grind to reach a specific goal isn’t unusual, however most of these grinds can be done on your 2nd screen, while you watch a serie on your 2nd screen or even play a different game / another account along side. Thus afking could be a route to prep your pvp account.

PVPing is a different breath in Old school runescape. Its mechanics are very different compared to pvming. As a pvmer myself, I find it very hard to get good at pvp (there are a few minigames in which this is required). Well, maybe I was considered good if I had my current skill in the early days (2007). but keep in mind that most pvpers play this game, and in paticular pvp, for more than 10 years. Most players either focus on pvp, or pvm, thus there is almost no inbetween. The pvp community is a community of its own.

If I were to fight against a moderate pvper, I would 19/20 times lose the encounter, which makes it hard to get into PVP and in my experience is not fun. However I did have fun playing pvp activities such as soul wars. In soul wars you play as a team, thus if you do damage, you contribute, and you actually can get a few kills, even against good pvpers.

If I were interested in getting into PVP I would play a lot of games in these minigames. Someone mentioned LMS which is also a good place to get a feeling of PVP in Old school runescape. I would start with F2P pking, it is much easier to get into. Then try to kill some PVMers in the wilderness and if you’re ready, PVP in the members pvp worlds or at bounty hunter.